M103 Distributor cap and rotor

Today I changed the Distributor cap and rotor in my 1986 300SE.  I bought the new cap and rotor about two years ago but only just got around to changing it.   Unlike on the earlier engines, the M103 distributor cap is a fiddly job.   Instead of sticking upwards, it comes out sideways as it attaches to the camshaft.

I find that whenever I buy one of these older cars, chances are the distributor cap needs changing.   It is not generally an expensive part on the mainstream engines, and can make a big difference to the smooth and reliable running of the engine.  An old and pitted cap is going to produce a weak, inconsistent spark.    I normally just purchase a cap and rotor when I buy the car.  On the off chance it doesn’t need replacing it is good to have a spare.

My 86 300SE is no exception.   The cap wasn’t in terrible shape but it was clearly worn and ready for replacement.  The contacts were worn and so was the rotor.

The M103 distributor cap differs from the other cars because of how it connects to the camshaft.  To remove the cap, there is a cover for the plug wires that snaps off.   Next, three 5mm allen bolts hold the cap to the engine.   Two of them are easy to see but the third one is underneath the cap and only just visible from the left hand side.   I found it easiest to remove it with an allen socket and a small extension.   Using a 1/4″ ratchet made it easier to clear the Serpentine belt.   I used an adapter for my 3/8″ socket into the smaller ratchet for this purpose.

It is easier to remove the plug wires first.  I found there wasn’t clearance to remove the cap until the plug wires were removed anyway – the fan gets in the way.    The actual distributor cap has a black cover that provides the mounting points for the plastic cover and splash protection for the ignition wires.   On the surface, mine looked ok but on closer inspection I would have preferred to change it.   It has a couple of small cracks in it.    In the picture above, the old M103 distributor cap on the right still has the cover attached.

Once the cap is removed, the rotor requires 3x3mm allen bolts to remove it.   In my case, one of the bolts was rounded off.   It was quick easy to remove with vice grips, at the expense of breaking the old rotor.

I gave the inner plate as good a clean as I could.   Ideally I would have replaced this too.  As can be seen in the picture above it has seen better days.

In total, the job took about an hour to do, including the rounded off bolt.   I had not changed an M103 distributor cap before, so it would likely be faster next time.      Note, that the M104 engine is very similar.   The M104 is basically a DOHC version of the M103.  More power, but also even more prone to eating head gaskets.